1. Field of the Invention
The present invention relates to a tire pressure gauge, more particularly to a tire pressure gauge having a retractable pen element which can be concealed within or exposed from a tubular housing.
2. Description of the Related Art
Conventional tire pressure gauges are generally provided with a pressure responsive movable indicator rod to indicate the pressure value that is detected thereby. Referring to FIG. 1, a conventional tire pressure gauge 1 comprises a tubular housing 10, a pressure measuring head 11 formed with a pressure inlet hole 11a, a piston 12 disposed movably in the tubular housing 10, a spring 13, a hollow guide 14, and a plunger or graduated pressure indicator 15. When measuring the pressure inside a tire (not shown), the pressure measuring head 11 is connected to an inflating valve (not shown) of the tire so that the gas in the tire enters the tubular housing 10 and pushes the piston 12, which in turns moves the pressure indicator 15 out of the tubular housing 10 against the biasing action of the spring 13. The pressure inside the tire is read from the graduation on the pressure indicator 15 which is aligned with a pointer, e.g., the bottom end of the hollow guide 14 or the bottom end of the tubular housing 10. After reading, the pressure indicator 15 is pushed inward manually to revert to its original position.
The tire pressure gauge 1 provides a single function of measuring the pressure inside a tire. Since a writing instrument is usually needed during measurement of the tire pressure for recording the detected pressure value, another conventional tire pressure gauge 2, which incorporates a writing instrument, has been proposed. Referring to FIGS. 2 and 3, the tire pressure gauge 2 also includes a tubular housing 24 and a plunger or graduated pressure indicating rod 22 provided in the housing 24. The pressure indicating rod 22 is formed with an axial blind bore for receiving a pen element 21. A coupling sleeve 23 is mounted on the pressure indicating rod 22 at the blind bore for coupling the pen element 21 to the pressure indicating rod 22. The pen element 21 has a writing tip 211 that extends outwardly of the coupling sleeve 23. In order to position the pen element 21 when the latter is in use, the pressure indicating rod 22 has a part formed with external screw threads 221 for engaging internal screw threads 241 on the tubular housing 24.
Although the tire pressure gauge 2 provides the combined functions of measuring the pressure inside a tire and providing a pen element for writing down the measured pressure value, the tire pressure gauge 2 suffers from the following drawbacks:
(1) Since the pressure indicating rod 22 engages threadedly the tubular housing 24, prior to using the tire pressure gauge 2 for measuring the pressure inside a tire, the pressure indicating rod 22 must be rotated to disengage the same from the tubular housing 24 so as to permit axial movement of the pressure indicating rod 22 relative to the tubular housing 24. PA1 (2) Since a writing tip of the pen element 21 is exposed from the pressure indicating rod 22, an additional cap 25 is needed to prevent ink of the pen element 21 from fouling the user's clothes and to prevent the sharp tip 211 of the pen element 21 from piercing the user's clothes. Moreover, recapping of the pen element 21 is required after use.